Two developments have in recent years brought on changes in the field of the orthodontist as to the manner of applying brackets to teeth for straightening them. Originally, metal bands were mounted and cemented about a tooth to be straightened. The metal band had attached at its front surface a metal bracket for receiving the arch wire that applied appropriate forces to the tooth in its straightening action. With new and stronger adhesives made available, it became possible to cement the bracket directly to the tooth. With the development of stronger plastics, orthodontists became able to eliminate the metal band and through the use of a clear plastic bracket provide for a more esthetic appearance with greater comfort to the users.
However, it became apparent that exact precision of results were not forthcoming with the use of plastic brackets for the reason that metal arch wires placed in the slotted portions formed in the plastic bracket did not provide the required precision and exactness of tooth movement and there was a loss of proper control of tooth position theretofore achieved by the combined metal brackets mounted on metal bands and use of metal arch wires. The reason for loss of control is due to the fact that plastics do not have the same strength and density of metal. Therefore, plastic brackets cemented on a person's teeth tend to abrade, stretch, give and become distorted as well as possibly fracture under the forces on an arch wire in attempting to straighten teeth orthodontically.
The tendency of the plastic to distort excessively and, in some cases, actually fracture when forces are applied via an arch wire attached thereto, was recognized in U.S. Pat. No. 3,930,311 issued Jan. 6, 1976. In an attempt to overcome said problem, said patent describes the use of a sheet metal stiffening core embedded in the plastic extending essentially normal to the longitudinal axis of the slot in the bracket so as to reinforce the wings projecting on either side thereof. The major portion of the exposed surfaces of the slot remain plastic while only a thin edge of the core is exposed as a narrow band bisecting the walls of the slot.
Unfortunately, the precise positioning of the cores as disclosed in said patent is difficult if not impossible to obtain in practice, and it is extremely difficult, if indeed possible, to mold plastic thereabout. Moreover, the major surface of the slot remains plastic and is still subject to the various deficiencies inherent in the material such as the susceptibility to abrasion, stretching and giving under the forces of the arch wire. An additional disadvantage with the structure described in said patent is that when transparent or translucent plastic is used the metal cores are readily visible in the wings on either side of the slot and detract from the unobtrusive characteristic of otherwise transparent plastic.